This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-143530, filed May 16, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to an LED intensity modulation type driving circuit for controlling emission/non-emission of an LED output in accordance with the high/low level of an input voltage pulse and, more particularly, to an LED driving circuit capable of outputting a high-speed modulated optical signal almost free from an emission pulse waveform distortion that is inevitable in optical data transmission of a high-speed optical data link.
Multimedia data, now widely-used and being increasingly developed, is typically exchanged over a variety of high-speed optical network backbones throughout the world. To be exchanged, optical transmitting and receiving modules play an important role as key components in optical communication systems of long-distance LAN connections, and short to mid-distance LANs supported by fiber-optic channels and Gigabit Ethernet. Purpose-built optical transmitting and receiving modules used in IT (Information Technology) systems currently in use have been developed at a cost, sacrificing versatility at the same time.
Recently, demands have arisen for a wide application range of optical interconnection techniques without limiting them to special applications such as high-performance optical communication and connection between computers, particularly server devices, in order to ensure connectability over long distances and high throughput even for exchange of data between multimedia devices such as home appliances, and to provide even end users with ease of use.
To meet these demands, IEEE 1394a extended from the electrical specification IEEE 1394 standardized in 1995 is further extended to optical applications, promoting standardization of IEEE 1394b which targets an optical data link using a POF (Plastic Optical Fiber) and is applicable to high-speed, low-cost, medium-distance connection. In the future IT field in a broad sense, practical performance is important as an interconnect requirements specification in addition to specifications which define basic transmission performance such as high throughput, regardless of the optical or electrical transmission signal form.
Strong electrical demands have arisen in terms of system mountability so as to realize low power consumption without cooling, have the same electrical interface as that of another IC used in an IT device, and if possible, obtain characteristics which allow operation at the same power supply voltage. Much lower cost than a conventional optical transmitting module is required in terms of cost performance.
The standard draft IEEE 1394b under examination adopts a gradient index plastic optical fiber having a large core system in order to reduce the cost of an optical fiber for use, simplify the internal structure, and reduce the cost of an optical link module itself. An example of this plastic optical fiber is combined with a red light source which falls within the low-loss wavelength region of the fiber.
The light source of the red wavelength region is a light-emitting diode (LED) which emits light in a wavelength region around 650 nm, or an optical semiconductor laser (LD) which oscillates at 650 nm. Of these light sources, the LD must be employed in terms of essential element response characteristics in high-speed S800 or more which is considered to be the mainstream of the optical data link as the technique to be established in the near future.
On the other hand, low-speed standards S100 to S400 will mainly adopt LEDs which can simplify the circuit arrangement and optical coupling system of an optical transmitter that are important factors for reducing the module cost.
In fact, products using LEDs as light sources are commercially available and widely used in an optical data link of several ten Mb/s or less that targets audio and FA systems.
FIG. 2 is a basic block diagram showing a conventional optical transmitting circuit.
As shown in FIG. 2, a constant current pulse prepared by ON/OFF-modulating, using a transistor switch, a DC current generated by a constant current source is generated, and the output is applied to a load LED. This method does not pose any technical problems in a low-speed link, up to about 10 Mb/s. In general, however, the switch response characteristic of an optical signal is low due to a large internal capacitance of a device that is a property unique to the LED. The low response characteristic determines the optical data link speed.
As one effective solution for relaxing this constraint, a peaking pulse current in phase with a constant current pulse is superimposed on an ON/OFF-modulated driving constant current pulse in synchronism with level inversion of the driving current switch, thereby accelerating the transient response of the LED.
If the means for compensating for and accelerating low-speed response characteristic is added, the original transmission data rate is as low as several tens of Mb/s, and the essential time constant which determines the device speed unique to the LED is as short as several ns or less. In addition, the signal processing speeds of photoelectric conversion and logic level conversion after optical transmission are substantially negligible with the development of the IC process technique. Optical transmission can be realized in which the signal error rate in data transmission is suppressed to be low so as not to cause any practical problems.
However, if optical transmission in which the transmission bit rate is increased to 100 Mb/s or more is to be realized by an LED along with the recent demands described above, the conventional method cannot be simply extended or applied.
As shown in FIG. 1, one of the present inventors has proposed a method capable of transmitting data at around 100 Mb/s by adopting the principle of current peaking driving, devising peaking superimposition, and decreasing the LED driving amplitude.
More specifically, a current bias required for an LED to generate an ON light intensity is always supplied from a DC constant current source to the LED anode. A CMOS buffer converts an external pulse signal input Vp into a rectangular pulse which changes at a low impedance between power supply voltages Vdd and Vss. A bias current input to the LED anode is ON/OFF-modulated by using the pulse. At the same time, a differential current flowing through a capacitance Cp is superimposed and supplied as a peaking current to the LED anode.
A method using an inductor instead of current peaking using a capacitance has also been proposed. The circuit shown in FIG. 1 that is driven by a conventional constant current switch is used as a main arrangement. A circuit constituted by series-connecting an inductor and resistor is parallel-connected to an LED, and they are driven as the entire load of the current output in place of using only the LED as a load. That is, a peaking current is generated by the inductor in transition and supplied to the LED.
Adding the means for superimposing the peaking current can shorten the transient response time of an output optical signal to some extent. However, effective peaking superimposition inhibits a peaking pulse from completing attenuation within the signal pulse width because of the time constant. A pulse tail or the like is generated, and the bit rate cannot be so increased. Further, the output optical pulse width essentially becomes smaller than the driving pulse width of an electrical signal.
The optical pulse width decreases by 1 ns or more in general, and in some cases by 10 ns, which depends on the LED driving circuit method. These values cannot be ignored when the minimum pulse width of a transmission optical signal is 10 ns or less. The decrease causes great variations in the ON/OFF pulse duty ratio of the transmission signal or additional increase in time jitter, seriously influencing the transmission waveform quality.
The phenomenon that an output optical pulse becomes narrower than a driving pulse input to an LED results from a property unique to a device in which an LED generates light almost proportionally to a forward current flowing through the diode. The electrical equivalent circuit model of the diode is expressed by a structure as shown in FIG. 24 in which a current flows through a series resistance Rs of the diode and a p-n junction diode capacitance Cd is parallel-connected to a constant current source Id.
An output current If from the constant current source Id depends on an intrinsic diode voltage Vd applied across Id or Cd. Letting Is be the saturation current of diode junction, N be the radiation coefficient, Vt=kT/q (k: Boltzmann constant, q: electron charge) be the thermal electromotive force with respect to an operation temperature T of the p-n junction, the first approximation is given by If=Isxc3x97exp[Vd/(Nxc2x7Vt)].
When the voltage Vd applied to the LED is a forward or reverse bias voltage which does not cause the LED to emit light, Cd increases depending on the voltage Vd, and substantially exhibits a value dominated by the p-n junction capacitance Cj. Under the operation condition where the LED emits light, the transition time capacitance component which increases in proportion to the forward current If is added.
A general LED has Rs of several xcexa9 to several tens of xcexa9 and has a value of several ten pF to several hundred pF only by Cj in zero bias. Thus, even when an ideal rectangular pulse Vh is input to the LED, as shown in FIGS. 25A and 25B, the internal intrinsic diode voltage Vd changes by an exponential attenuation function with a time constant (Rsxc2x7Cd) of a little less than 1 ns to about 10 ns, and exhibits a waveform which is asymptotic to a steady-state value Vhigh upon the lapse of time. The forward current If exponentially increases with respect to Vd, as represented by the above equation. The forward current If reaches the same current value as the steady-state value only when the forward current If falls within (Nxc2x7Vt) with respect to the value Vd at which a desired maximum steady-state current is obtained, e.g., comes near to, e.g., 30 mV to 50 mV.
In other words, for a rectangular pulse externally input to the LED, a current starts flowing when the voltage reaches a peak value, and the LED outputs an optical output in proportion to the forward current value flowing through the LED. At the pulse leading edge, the rise of the emission pulse greatly delays from the input rectangular pulse. At the pulse trailing edge, the fall of the pulse hardly delays because the current flowing through the LED abruptly decreases when the voltage drop of Vp applied to the intrinsic diode reaches several ten mV. The difference between the delay times in transition makes an optical output pulse width smaller than an input electrical pulse signal.
If a peaking current is superimposed, like the prior art, in order to decrease the signal delay of the transient response, the charge/discharge time with respect to Cd can be shortened to shorten the delay time. Generally in superimposition of a peaking current, a current exceeding the steady-state peak value of If necessary for obtaining an optical signal amplitude is supplied. Current consumption in operation increases, and the optical output waveform itself has peaking and deforms from the rectangle. Consequently, the discrimination level of the high-speed receiving circuit varies, and the time jitter of a reception output signal increases.
To the contrary, an LD used in high-speed optical communication can obtain laser oscillation by limiting the space area to a narrow one within the element and realizing a region having a large optical amplification gain. The total area occupied by the element itself to be substantially operated suffices to be small. The unique capacitance value of the element is 10 pF or less at most.
As for an LD output, a large optical output can be obtained with high photoconversion efficiency in proportion to the difference from the threshold of an injection current to the element at the boundary of a current having a given threshold or more. It is, therefore, effective to turn on/off a constant current source as a reference at a high speed and convert a current into a current pulse. This method has been applied to most LD driving circuits.
Especially, high-frequency characteristics can be relatively easily ensured because the LD can employ a normal high-frequency circuit technique in which even when the output terminal of a current switch circuit and the LD are apart from each other, they are connected via a transmission line of 50 xcexa9 to match all impedances. To the contrary, the LED can always obtain an optical output proportional to an input current regardless of the magnitude of the injection current, but is essentially difficult to realize high-frequency modulation using an electrical signal because a large capacitance is added.
It is an object of the present invention to provide an LED driving circuit capable of shortening the transient response time, realizing high-speed modulation, minimizing an increase in current consumption, and obtaining output light almost free from the pulse waveform distortion of an emission signal by a new driving method based on an operation model even in an LED having a large internal capacitance as a unique device characteristic, and an optical transmitting module capable of realizing low power consumption, low cost, and high-speed transmission by using the LED driving circuit.
An LED driving circuit according to the present invention comprises a voltage generation circuit for generating a first output voltage of a low level and a second output voltage of a high level, a first MOS switch for transferring the first output voltage to an output terminal, a second MOS switch for transferring the second output voltage to the output terminal, and a pulse generation circuit for shaping a waveform of an externally input signal and generating first and second rectangular pulses having a complementary relationship, wherein an LED is electrically connected to the output terminal, the first rectangular pulse is input to a gate of the first MOS switch, the second rectangular pulse is input to a gate of the second MOS switch, the high level is determined by a forward peak current or forward voltage of the LED which is necessary for outputting light of a predetermined intensity from the LED, and the low level is set to a voltage value for changing an emission intensity of the LED to zero or a negligible value.
An LED driving circuit according to the present invention comprises a voltage generation circuit for generating a first output voltage of low level, a second output voltage of a first high level, and a third output voltage of a second high level which is higher than the first high level, a first MOS switch for transferring the first output voltage to an output terminal, a second MOS switch for transferring the second output voltage to the output terminal, a third MOS switch for transferring the third output voltage to the output terminal, and a pulse generation circuit for shaping a waveform of an externally input signal and generating first, second, and third rectangular pulses, wherein an LED is electrically connected to the output terminal, the first rectangular pulse is input to a gate of the first MOS switch, the second rectangular pulse is input to a gate of the second MOS switch, the third rectangular pulse is input to a gate of the third MOS switch, after the third rectangular pulse changes to the high level, the second rectangular pulse successively changes to the high level, the first rectangular pulse has a complementary relationship with an OR value of the second and third rectangular pulses, and a high-level voltage applied to the LED is binary.
An optical transmitting module according to the present invention comprises an IC having the above-described LED driving circuit, an LED connected to an output terminal of the LED driving circuit, a submodule on which the IC and the LED are mounted, an optical connector which is optically coupled to the LED, a lead which is electrically coupled to the IC and the LED, and a package for housing the IC, the LED, the submodule, the optical connector, and the lead.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.